Types of Producer.—In the suction type, air and steam are drawn through the fuel bed by the suction of the engine, or by an exhaust fan which afterward delivers the gas to the engine under a slight pressure. The former arrangement is used for small plants only. The latter permits of cleaning while running (therefore of continuous operation) and is the most popular type today. Pressure producers use forced draft blowers. Their operation is attended with some discomfort and they must be shut down for thorough cleaning. Combination producers employ both pressure and
11,000 12,000 13,000 14,000 B.iu.perlb.of Dry Coal
FIG. 27.—Gas producer characteristic curves.
suction, atmospheric pressure being maintained above the fuel bed. Up-draft and down-draft producers are defined by their names. Double-zone producers have down-draft in the upper half, up-draft in the lower half and the gas discharge at mid-height.
Capacity.—Rates of gasification are high when temperatures are high: i.e., when the cold gas efficiency is low. Average rates in suction producers are 5 to 20 Ib. per square foot of fuel bed horizontal cross-section area per hour. A mean rate of 10 Ib. may be expected for anthracite coal.
Fuels.—Success may be expected with confidence when anthracite (pea or No. 1 buckwheat) is used. The factors of chief importance are moisture, ash, tar, clinkering and heat value. Soft coal is used with success in down-draft or double-zone producers,